Conventionally, a mobile communications system using the CDMA scheme, the levels of signals received by a base station from mobile stations vary greatly depending on where the mobile station is located, and thus the far-near problem occurs that a received signal having large power interferes with a received signal having small power. In particular, in a communication system using the CDMA scheme, a plurality of mobile stations share the same frequency band, and hence for each mobile station, signals from other mobile stations become interference signals and degrade the communication quality of its channel. Such interference associated with the far-near problem leads to the reduction of the number of multiple stations in the same cell.
One method for solving this problem is a transmit power control circuit disclosed in Laid-Open Japanese Patent Publication No.2001-69073. In this transmit power control circuit, information data subjected to transmit power control and transmit power control information not subjected to transmit power control are transmitted at the same time from a base station to a mobile station, and the mobile station determines the transmit power of a signal to be sent in return to the base station based on the transmit power control information from the base station.
The transmit power is determined by estimating the receive level of the signal from the base station, and, based on a result of reduction in this receive level, selecting a transmit power for a signal to be sent in return to the base station.
When the receive level decreases at a mobile station, the mobile station selects the transmit power of the signal sent at the previous time in return to the base station under transmit power control as a transmit power for a signal to be sent in return to the base station.
A typical transmit power control apparatus thus controls the transmit power using only the receive level of the received signal at the mobile station, so that it is effective to a certain degree in controlling the transmit power against instantaneous reduction in the level of the received signal due to fading and the like and interference because of. However, changing the transmit power more than needed in response to instantaneous reduction in the level of the received signal and interference causes the transmit power value to vary suddenly and greatly, and consequently the problem occurs that the closed-loop transmit power control is affected in a harmful way.